1. Field of the Invention
The present invention relates to a method of manufacturing a metallic decorative member by injecting molten molding material into a cavity formed by a set of molding dies from a plurality of gates provided at the cavity.
2. Description of the Related Art
FIG. 7 shows a conventional speedometer 1 mounted to an automobile and such, in which the speedometer 1 is provided with an annular decorative member 3 arranged at a boundary of a facing plate 2 and a speedometer dial plate 4. In order to eliminate a process step of coating for obtaining a luxury texture to reduce the cost, the decorative member 3 described above is produced by injection molding using a mixture of metal particles and synthetic resin as molten molding material.
For an injection-molded article such as the metallic decorative member described above, there is a problem of producing a linear scar called a weld line on a surface of the injection-molded article. The mechanism of production of the weld line is described below in reference with FIG. 8A and FIG. 8B.
As shown in FIG. 8, molten molding material 20 which is a melted form of molding material, which is a mixture of synthetic resin 21 and metal particles 22, is injected into a cavity 13 formed by a set of molding dies 11, 12 from a plurality of gates (not shown) of the cavity 13. As the molten molding material 20 flows in the cavity 13, the metal particles 22 included in the molten molding material 20 align at a surface of the molten molding material 20 contacting the molding dies 11, 12 along a direction parallel to the flow of the molten molding material 20. Then, as the injections of the molten molding material 20 from the plurality of gates continues, heads S1, S2 of flows of the molten molding material 20 injected from the different gates move towards each other and contact (merge) each other, as shown in FIG. 8B. At this point, since the injections of the molten molding material 20 from the plurality of gates are performed in a similar manner for each gate, dynamic pressure of the head S1 and dynamic pressure of the head S2 are equal with respect to each other, thus the flows of the molten molding material 20 stop after the respective heads S1, S2 have contact each other. Thus, the heads S1, S2 are stopped before the completion of the alignment of the metal particles 22, so the metal particles 22 remain unaligned. Such unaligned orientation of the metal particles 22 causes production of the weld line on a surface of the injection-molded article.
Japanese Patent Application Publication No. 2002-292703 discloses a method using the injection molding for preventing the production of the above-described weld line. In this method, firstly a die cavity is filled with molten molding material including metal particles, and then the molten molding material is forced to move within the die cavity. According to this method, by forcing the filled molten molding material to move within the cavity, the metal particles at a contacting portion of heads of flows of the molten molding material can be aligned at a surface of the molten molding material along a direction of movement of the molten molding material. As a result, the production of the weld line can be prevented.